RSRRacer

Recently, I have had the misfortune of my engine "letting go." <img src="graemlins/cussing.gif" border="0" alt="[grrrrrrr]" /> The engine was (and will again be) a 3.3L race motor with relatively high compression (almost 12:1) twin crankfire plugs, etc.

Occasionally the car would lose power due to what I am believing to be a fuel delivery problem. The loss of power would last only 1/2 or 1 sec, and would be accompanied by a change in engine note. Also occured more often on a light fuel load.

For those familiar with VIR, the problem would happen in 1) bottom of Hogpen, 2) just before Oak Tree, 3) in the short straight before the lower esses, and 4) just after as I make the sharp right at the end of the back straight just before I am going down the Roller Coaster. All of these are high G right turns, which follow high G left turns by about 3-6 seconds.

If there is a fuel pickup problem, is it more likely that the problem is actually caused in the LEFT hander and does not manifest iteself until 3-6 seconds later (when I happen to be in a right hand turn?) Another way of asking is how long would it take the fuel shortage to get from the tank to the carbs and result in the loss of power and flat engine note.

Regardless of when the problem occured, would a fuel shortage which lasts .5 - 1 second damage the engine by making it run "lean" for a short while?

Thanks for any help.

E. J. - 993 Alumni

Chris, Wouldn't the fuel cell solve this problem and make your question moot?

Maybe 3k worth of future insurance?

Talk to you this week.

E. J.

David Salama

Chris,

I am not familiar with your engine, but I have highly modified my 944 Turbo including a Mass Air Flow with ARC II fuel controls, and ARM 1 Air Fuel Ratio meter. I have my air fuel ratio meter within my site at all times to make sure I'm never running lean. The monitor is cheap (about $100), and easily hooks up to the O2 sensor wires. This is truly cheap insurance. Good luck.

Dave
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Steve Weiner-Rennsport Systems

Hi Chris:

Sorry to hear of your engine troubles. This is not unfortunately, an insolated occurance.

The direct answer to your question would be that how well any engine tolerates a drop in fuel pressure & volume is dependent upon how long this happens for, and how lean or rich it is. For sure, 12:1 motors really need a bulletproof fuel supply and are intolerant of such things.

FWIW, carbs do run for a 1/2 sec well enough unless the floats are low and its jetted rather lean.

For all race cars, we use either multiple pickups inside the fuel cell, or incorporate a surge tank in the fuel system plumbing to prevent this from happening. Voila', no more problems.

RSRRacer

Thanks for the info from all of you. Except for EJ, who will continue to sit back in his Motronic bliss whilst I curse the tops of the six velocity stacks staring up at me from the garage floor.

I already have a fuel cell (uninstalled as of yet) and also have the meter which connects to the 02 sensor.

So, from the responses, am I correct to assume that indeed a .5 to 1 second loss of fuel COULD have caused this problem, or contributed to the problem?

Also it is safe to say that if this indeed caused the failure, wouldn't the piston and chamber (or the remnants thereof) show signs of detonation?

Thanks. EJ I do want to talk if you can shed some light. I still need to run this by Jim.

BrianKeithSmith

Chris:

I feel your pain man.

I broke a rocker arm at CMP a week ago last Friday.

Jim is working on my car - at the same time he's working on yours.

Brian

JPIII

Webers have been known to uncover the intakes for the main jets (gas in the float bowl)in high side load situations. There is a mod in BA's book to minimize this problem. The effects would be about what you describe, me thinks. As Mr. Weiner has said, float levels are also critical.

Detonation is a piston killer. .5 sec is about 30 cycles at 7K......enuff to kill, me thinks.

Steve Weiner-Rennsport Systems

[quote]Originally posted by Chris Brown:
<strong>Thanks for the info from all of you. Except for EJ, who will continue to sit back in his Motronic bliss whilst I curse the tops of the six velocity stacks staring up at me from the garage floor.

(snip)
So, from the responses, am I correct to assume that indeed a .5 to 1 second loss of fuel COULD have caused this problem, or contributed to the problem?

Also it is safe to say that if this indeed caused the failure, wouldn't the piston and chamber (or the remnants thereof) show signs of detonation?

Thanks. EJ I do want to talk if you can shed some light. I still need to run this by Jim.</strong><hr></blockquote>

Hi Chris:

Yessir, loss of fuel pressure at high RPM & load in an engine configured like yours, could be detrimental to the health of its internals,...

Like J.P. said, Weber float chambers (unmodified) can "unport" and starve a cylinder of fuel. Don't forget that on each carb, there are two floats and that the pair of cylinders that are fed by a single float chmaber draws down much quicker than the single-float-per-cylinder ones.

Exactly what failed in your engine? Detonation can be detected by close, very detailed examination of plugs and pistons and at high load and that CR, it happens VERY quickly.

BryanMD

I feel for your grief and will add some tidbits.

If your engine failed due to detonation the pistons will tell the tail. The plugs will as well, but with pistons in hand it will be obvious if the problem was detonation. Wether this did the engine in or not, your performance complaint is likely starvation.

The carbs certainly will run a short while at full throttle before leaning out. The bowl mods can help, and there are few reasons not to apply them, but they are not a cure-all.

Most air-fuel meters are run by a standard Oxygen sensor which is limited in reaction time. It may not tell you soon enough to make a difference, and then only if you are looking at it and can shut down in time. If you are driving near 100%, do you want to make time for one more thing to watch (a lot)? There are faster systems but you still have to be watching and then add reaction time.

I have personally run a 3.6L with 12.5:1, twin plug crank fire and Weber 46's with the bowl mods applied. I never had the problems you have described. I also had two things it appears you did not. A fuel cell (by itself not a cure, just a safety item) and an external surge tank (most likely part of the cure). I use a Carter 100 gph pump as a "pre-pump" between the cell and surge tank and a Bosch high pressure pump between the surge tank and the engine. The pressure is maintained by a recirculating regulator returning fuel to the surge tank with overflow back into the cell. The system is designed to run electronic injection, the carbs do not run out of fuel! Get yourself one and don't worry about that part again, VIR is too much fun to worry.